Chlorine-Rich Na6−xPS5−xCl1+x: A Promising Sodium Solid Electrolyte for All-Solid-State Sodium Batteries

Developing argyrodite-type, chlorine-rich, sodium-ion, solid-state electrolytes with high conductivity is a long-term challenge that is crucial for the advancement of all-solid-state batteries (ASSBs). In this study, chlorine-rich, argyrodite-type Na6−xPS5−xCl1+x solid solutions were successfully developed with a solid solution formation range of 0 ≤ x ≤ 0.5. Na5.5PS4.5Cl1.5 (x = 0.5), displaying a highest ionic conductivity of 1.2 × 10−3 S/cm at 25 °C, which is more than a hundred times higher than that of Na6PS5Cl. Cyclic voltammetry and electrochemical impedance spectroscopy results demonstrated that the rich chlorine significantly enhanced the ionic conductivity and electrochemical stability, in addition to causing a reduction in activation energy. The Na5.5PS4.5Cl1.5 composite also showed the characteristics of a pure ionic conductor without electronic conductivity. Finally, the viability of Na5.5PS4.5Cl1.5 as a sodium electrolyte for all-solid-state sodium batteries was checked in a lab-scale ASSB, showing stable battery performance. This study not only demonstrates new composites of sodium-ionic, solid-state electrolytes with relatively high conductivity but also provides an anion-modulation strategy to enhance the ionic conductivity of argyrodite-type sodium solid-state ionic conductors.


Introduction
Sodium-ion batteries (SIBs) have been considered a highly promising alternative to commercialized lithium-ion batteries (LIBs) due to the natural abundance of sodium resources (2.64 wt% for Na against 0.0017 wt% for Li) and their higher safety levels with similar charge/discharge mechanism [1].Unfortunately, conventional SIBs face environmental and safety issues originating from the effumability and leakage of flammable organic liquid electrolytes [2,3].Employing solid-state electrolytes (SSEs) instead of liquid organic electrolytes is a viable solution to the battery issues, since inorganic SSEs are non-volatile and non-flammable [4][5][6].Therefore, the rational design of advanced sodium SSEs has been treated as a reliable way to achieve high-performance SIBs.
Argyrodite-type conductors with high ionic conductivity and stable interfacial properties have been considered as promising electrolytes for all-solid-state batteries due to the absence of transition-metal species [38,39].The Li 6 PS 5 X lithium-based conductor (X = Cl, Br, and I) showed their promising properties as an SSE, whereas few reports are available on its sodium analogs.Nevertheless, in previous reports, an argyrodite-type conductor, Na 6 PS 5 Cl, was synthesized [40].Unexpectedly, the ion conductivity of Na 6 PS 5 Cl was only 1.2 × 10 −5 S/cm, which was significantly lower than that of its Li 6 PS 5 Cl analog (3 × 10 −3 S/cm) [41].The electrochemical window of Na 6 PS 5 Cl is only 1.3 V, which makes it impossible for practical use.Increasing the degree of structure disorder by raising the halide content of electrolytes can strengthen the diffusivity of ions in the SSE [41,42].Therefore, increasing the Cl and Na vacancy contents in Na 6 PS 5 Cl is favorable for the promotion of Cl − /S 2− disordering, as well as Na-ion mobility.
In this work, a highly conductive chlorine-rich sodium solid electrolyte, Na 5.5 PS 4.5 Cl 1.5 , was successfully synthesized by increasing Cl and Na vacancy contents with an outstanding conductivity of 1.2 × 10 −3 S/cm at room temperature, which is almost ten-fold greater than that of pristine Na 6 PS 5 Cl.The introduction of Cl and Na vacancy contents in Na 6 PS 5 Cl induced a significant disordering of S 2− /Cl − arrangement, leading to the lowering of the activation barriers.Moreover, the electrochemical window of the Cl-rich SSE increased to 2 V, which makes the SSE a practical candidate for ASSBs.We demonstrated the availability of the chlorine-rich Na 5.5 PS 4.5 Cl 1.5 SSE by assembling a Na 3 V 2 (PO 4 ) 3 //Na 5.5 PS 4.5 Cl 1.5 //Na full battery, indicating the practical application value of the new Na 5.5 PS 4.5 Cl 1.5 sulfide electrolyte for ASSBs.

Materials Synthesis
Na 6−x PS 5−x Cl 1+x (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) was synthesized by a hightemperature, solid-state reaction method.Stoichiometric Na 2 S, P 2 S 5 , and NaCl (all chemicals with a purity of 99.99%; Macklin Biochemical Co., Ltd., Shanghai, China) were weighed by chemical stoichiometry in an Ar-filled glovebox (<1 ppm of O 2 , H 2 O) and sealed in a 45 mL ZrO 2 pot with 15 ZrO 2 balls with 10 mm diameters and 40 ZrO 2 balls with 5 mm diameters.The raw materials were ball-milled with a planetary ball mill apparatus (YXQM-0.45L,Changsha Mitrcn Instrument Equipment Co., Ltd., Changsha, China) at a rotational speed of 400 rpm for 8 h.Then, the obtained precursors were pelleted at 20-30 MPa in an Ar-filled glovebox (<1 ppm of O 2 , H 2 O), sealed in an evacuated Pyrex tube at pressures below 2 × 10 −3 Pa, and sintered at 450 • C for 12 h with a heating rate of 2 • C/min.After naturally cooling to room temperature, the as-synthesized Na 6−x PS 5−x Cl 1+x was transferred to a glovebox and mixed by hand with mortar and pestle for 15 min for further characterization and battery assembly.

Methods
Powder XRD measurements were conducted using a diffractometer equipped with CuKα 1 radiation (XRD-6010, Shimadzu, Tokyo, Japan) to ascertain the phase compositions of the synthesized powders.Prior to XRD measurements, samples were prepared in a glove box and sealed with polyimide film (Dongguan Meixin Co., Ltd., Dongguan, China) to shield them from moisture during the measurements.The diffraction data were collected in the 2θ range of 15 • to 50 • with step widths of 0.01 • .The morphology and elemental mapping analyses for the composite powders were obtained using energy-dispersive X-ray spectroscopy (EDS, Bruker QUANTAX, Berlin, Germany), and the obtained samples were examined using a scanning electron microscope (SEM) (JEOL Ltd., JSM-6390, Tokyo, Japan).
The ionic conductivities of the cylindrical solid electrolyte (SE) pellets were assessed using the AC impedance technique with a cell (stainless steel/SE/stainless steel), employing pressurizable sealing molds and measured between 25 and 85 • C.This measurement was repeated two or three times, applying 15 mV in the frequency range of 7 MHz to 1 Hz and utilizing a potentiostat electrochemical interface (Bio-Logic SAS, VSP-300, Claix, France).SE pellets with a diameter of 10 mm were fabricated under a pressure of 303 MPa using a polyaryletherketone mold.Electrochemical window measurements were conducted using sodium/solid electrolyte/sodium cells.Typically, 100 mg of SE was pressed at 150 MPa to form a solid pellet.Na metal foil (purity: 99.9%, thickness: 0.1 mm, diameter: 5 mm; 99.9%, MTI Corporation, Shenzhen, China) was placed on the top side of the electrolyte pellet and further pressed at 150 MPa.The electrochemical stability of Na 6−x PS 5−x Cl 1+x series was determined over a wider potential window, and a cyclic voltammetry (CV) test was conducted at a scan rate of 0.1 mV/s between −2.5 and 2.5 V for comparison of the voltage stability region.The stack pressure applied during the tests was about 5 MPa.
An all-solid-state battery employing the obtained chlorine-rich sodium-ionic conductor with the highest ionic conduction as the electrolyte was assembled under a pressure of 6 MPa within an Ar-filled glovebox to evaluate its charge-discharge performance.The battery assembly method and the utilized cathode materials were akin to those documented in previous reports [43].The details are as follows: Na metal foil served as the anode, while a composite mixture comprising Na 3 V 2 (PO 4 ) 3 (NVP) and Na 5.5 PS 4.5 Cl 1.5 (NPSC-6) (NVP:NPSC-6 = 7:3) was employed as the cathode.Prior to use, a sieve with a 10 µm mesh was utilized to eliminate large-particle-size powder in Na 5.5 PS 4.5 Cl 1.5 .A pellet with a diameter of 10 mm was produced by cold pressing approximately 100 mg of Na 5.5 PS 4.5 Cl 1.5 powder.Subsequently, Na foil (purity: 99.9%; thickness: 0.1 mm; diameter: 5 mm; 99.9%; MTI Corporation, Shenzhen, China) with a Cu mesh (purity: >99.9%; thickness: 0.045 mm; diameter: 8 mm; pore size: 0.4 mm × 1.5 mm; MTI Corporation, Hefei, China) was successively pressed onto one side of the pellet.On the opposite side of the pellet, the cathode composite (8 mg) with Al mesh (purity > 99.9%; thickness: 0.055 mm; diameter: 10 mm; pore size; 0.4 mm × 1.5 mm; MTI Corporation, Hefei, China) and an Al foil current collector (thickness: 0.1115 mm; diameter: 10 mm; purity: >99.9%,MTI Corporation, Hefei, China) were then pressed.Charge-discharge measurements were conducted at 25 • C, between 1.4 and 1.8 V, and with a current density of 0.03 mA cm −2 (0.05 C) using the LANHE CT2001A charge-discharge system (Wuhan LAND Electronics Co., Wuhan, China).The stack pressure applied during the tests was about 5 MPa.

Physical Characterization of Na 6−x PS 5−x Cl 1+x
Na 6−x PS 5−x Cl 1+x (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, or 0.6, denoted as NPSC-0, NPSC-1, NPSC-2, NPSC-3, NPSC-4, NPSC-5, and NPSC-6, respectively) samples were prepared by a solid-state reaction.The XRD patterns of the NPSC-0 sample were similar to those of the related argyrodite-type Cu 6 PS 5 Br, which displayed a monoclinic phase with a Cc space group [44,45].A structural model of atomic Na 6 PS 5 Cl was simulated by replacement of Cu and Br in Cu 6 PS 5 Br with Na and Cl through structural refinement using Vesta software (version 3) [46], as displayed in the inset of Figure 1a.Two different Na + sites were tetrahedrally coordinated.The Na1 site was surrounded by four S atoms, while the Na2 site was encircled by three S atoms and one Cl atom.The obtained NPSC-0 product was a solid block with chartreuse color (Figure 1b), while the outer and inner layers turned yellow in the NPSC-5 sample (Figure 1c,d).Comparisons of XRD patterns with those of other related compounds (such as tetragonal Na 3 PS 4 , cubic Na 3 PS 4 , and NaCl) are displayed in Figure S1, proving that no Na 3 PS 4 or NaCl impurities were included in the NPSC-0 sample.With the increase in x in Na 6−x PS 5−x Cl 1+x , the (400) peak gradually shifted to lower 2θ angles, while the ( -222) peak migrated to higher 2θ angles, indicating the formation of solid solutions in Na 6−x PS 5−x Cl 1+x .Further introduction of additional chlorine into the Na 6−x PS 5−x Cl 1+x structure caused exsolvation of NaCl impurity at x = 0.6 (Figure S2), resulting a decrease in ionic conductivity.Thus, under the present experimental synthesis conditions, it was shown that Na 5.5 PS 4.5 Cl 1.5 (x = 0.5) is the limit of solid solution formation in the Na 6−x PS 5−x Cl 1+x system.In other words, Na 6−x PS 5−x Cl 1+x formed solid solutions in the range of 0 ≤ x ≤ 0.5.
[44 ,45].A structural model of atomic Na6PS5Cl was simulated by replacement of Cu and Br in Cu6PS5Br with Na and Cl through structural refinement using Vesta software (version 3) [46], as displayed in the inset of Figure 1a.Two different Na + sites were tetrahedrally coordinated.The Na1 site was surrounded by four S atoms, while the Na2 site was encircled by three S atoms and one Cl atom.The obtained NPSC-0 product was a solid block with chartreuse color (Figure 1b), while the outer and inner layers turned yellow in the NPSC-5 sample (Figure 1c,d  The morphology analysis results showed that the particle sizes of the NPSC-0, NPSC-1, NPSC-2, NPSC-3, NPSC-4 and NPSC-5 samples were in the range of 5 to 50 µm, which was gradually reduced with the increase in the Cl -/S 2-ratio, as shown in Figures 2a and  The morphology analysis results showed that the particle sizes of the NPSC-0, NPSC-1, NPSC-2, NPSC-3, NPSC-4 and NPSC-5 samples were in the range of 5 to 50 µm, which was gradually reduced with the increase in the Cl − /S 2− ratio, as shown in Figures 2a and S3

Electrochemical Performance of Na6−xPS5−xCl1+x Electrolyte
The electrochemical stability of Na6−xPS5−xCl1+x was determined for comparison of the voltage stability region, the results of which are shown in Figure 3.The NPSC-0 sample showed a small stable region only at 1.33 V (Figure 3a).With the increase in x in Na6-xPS5-

Electrochemical Performance of Na 6−x PS 5−x Cl 1+x Electrolyte
The electrochemical stability of Na 6−x PS 5−x Cl 1+x was determined for comparison of the voltage stability region, the results of which are shown in Figure 3.The NPSC-0 sample showed a small stable region only at 1.33 V (Figure 3a).With the increase in x in Na 6−x PS 5−x Cl 1+x , the voltage stability region widened gradually.When x = 0.1, the stable region was at 1.52 V; when x = 0.2, the stable region shifted to 1.68 V.It kept increasing to 1.79 and 1.91 V in the NPSC-3 and NPSC-4 samples, respectively.The NPSC-5 sample exhibited a maximum stable voltage of 2.02 V, indicating that promoting the Cl content in Na 6−x PS 5−x Cl 1+x favors the widening of the operating potential window.
S3.The elemental mapping analyses shown in Figure 2b-f indicate the presence and uniform distribution of Na, P, S, and Cl elements within the NPSC-5 sample.

Electrochemical Performance of Na6−xPS5−xCl1+x Electrolyte
The electrochemical stability of Na6−xPS5−xCl1+x was determined for comparison of the voltage stability region, the results of which are shown in Figure 3.The NPSC-0 sample showed a small stable region only at 1.33 V (Figure 3a).With the increase in x in Na6-xPS5-    Figure 4 illustrates the complex impedance spectra of the uniaxially cold, isostatic pressed powder samples obtained in the study.The impedance plots exhibit a semicircle pattern accompanied by a spike observed at high and low frequencies, respectively [43].These features correspond to the total resistance (combining bulk and grain boundary contributions) and electrode resistances [47].The semicircle represents ionic transport within the materials, while the spike is indicative of ion-blocking behavior at the interfaces between the electrodes and electrolyte.The equivalent circuits used for impedance analysis, as depicted in Figure S4, were derived from the ZView program [47].Additionally, the semicircles observed at higher frequencies, displaying capacitances of 10 −10 F, suggest total resistance, with a combined contribution from both bulk and grain boundaries [48].
Materials 2024, 17, x FOR PEER REVIEW 6 of 11 pattern accompanied by a spike observed at high and low frequencies, respectively [43].These features correspond to the total resistance (combining bulk and grain boundary contributions) and electrode resistances [47].The semicircle represents ionic transport within the materials, while the spike is indicative of ion-blocking behavior at the interfaces between the electrodes and electrolyte.The equivalent circuits used for impedance analysis, as depicted in Figure S4, were derived from the ZView program [47].Additionally, the semicircles observed at higher frequencies, displaying capacitances of 10 −10 F, suggest total resistance, with a combined contribution from both bulk and grain boundaries [48].The ionic conductivity of Na6−xPS5−xCl1+x as analytical material was characterized using the AC impedance method.All the impedance spectra contained formed semicircle pattern at high frequencies attributed to the charge-transfer resistance and an inclined line at low frequencies associated with diffusion processes in the electrodes.With the increase in x in Na6−xPS5−xCl1+x (0 ≤ x ≤ 0.5), the semicircle diminished gradually, leading to a reduction in resistances by introducing rich Cl contents and facilitating Na + diffusion in Na6−xPS5−xCl1+x (Figure 4a).The calculated ionic conductivities rose gradually with increasing content of Cl, namely 9.4 × 10 -5 S/cm for NPSC-0, 1.4 × 10 -4 S/cm for NPSC-1, 2.1 × 10 -4 S/cm for NPSC-2, 4.2 × 10 -4 S/cm for NPSC-3, 7.9 × 10 -4 S/cm for NPSC-4, and 1.2 × 10 -3 S/cm for NPSC-5.The ionic conductivity of the NPSC-5 sample (Na5.5PS4.5Cl1.5 with x = 0.5) showed the highest ionic conduction, which was over a hundred times higher than that of Na6PS5Cl (1.0 × 10 -5 S/cm [40]), proving the importance of increasing the Cl content for Na + migration.The NPSC-06 sample displayed a lower ionic conductivity value (2.9 × 10 -4 S/cm) than that of NPSC-5 (Figure S4), which could be attributed to the low ionic conductivity of NaCl impurity (Figure S2).The whole testing and disassembly process of the The ionic conductivity of Na 6−x PS 5−x Cl 1+x as analytical material was characterized using the AC impedance method.All the impedance spectra contained formed semicircle pattern at high frequencies attributed to the charge-transfer resistance and an inclined line at low frequencies associated with diffusion processes in the electrodes.With the increase in x in Na 6−x PS 5−x Cl 1+x (0 ≤ x ≤ 0.5), the semicircle diminished gradually, leading to a reduction in resistances by introducing rich Cl contents and facilitating Na + diffusion in Na 6−x PS 5−x Cl 1+x (Figure 4a).The calculated ionic conductivities rose gradually with increasing content of Cl, namely 9.4 × 10 −5 S/cm for NPSC-0, 1.4 × 10 −4 S/cm for NPSC-1, 2.1 × 10 −4 S/cm for NPSC-2, 4.2 × 10 −4 S/cm for NPSC-3, 7.9 × 10 −4 S/cm for NPSC-4, and 1.2 × 10 −3 S/cm for NPSC-5.The ionic conductivity of the NPSC-5 sample (Na 5.5 PS 4.5 Cl 1.5 with x = 0.5) showed the highest ionic conduction, which was over a hundred times higher than that of Na 6 PS 5 Cl (1.0 × 10 −5 S/cm [40]), proving the importance of increasing the Cl content for Na + migration.The NPSC-06 sample displayed a lower ionic conductivity value (2.9 × 10 −4 S/cm) than that of NPSC-5 (Figure S4), which could be attributed to the low ionic conductivity of NaCl impurity (Figure S2).The whole testing and disassembly process of the steel/NPSC-5/steel configuration was completed in one shot, which can be observed in Video S1.
All the obtained samples exhibited linear relationships with ionic conductivity in the range of 25-85 • C. Figure 5a illustrates the temperature dependence of ionic conductivity.All obtained samples exhibited linear relationships between ionic conductivity and reciprocal temperature within the studied temperature range, indicating adherence to the Arrhenius law.By applying the Arrhenius equation (σT = σ 0 exp(−Ea/k B T), where σ 0 references a pre-exponential factor) [49], the activation energy (E a ) of Na 6−x PS 5−x Cl 1+x (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) for total conduction was determined from the slope of the plots.In Figure 5b, the composition dependence of the samples' activation energies (E a ) are depicted.AC impedance analysis results revealed that the ionic conductivity of all chlorine-rich Na 6−x PS 5−x Cl 1+x samples was higher than that of the primordial Na 6 PS 5 Cl (x = 0) sample, with ionic conductivity increasing from x = 0 to 0.5.These findings indicate that a rise in chlorine contents can enhance the ionic conductivity of Na 6 PS 5 Cl with a decrease in E a .Table S1 summarizes the ionic conductivities and E a values reported in the literature, as well as the values obtained in this study.Compared with the reported Na 6 PS 5 Cl (0.01 × 10 −3 S/cm) [40], the modified Na 5.5 PS 4.5 Cl 1.5 exhibited superior ionic conductivity of 1.2 × 10 −3 S/cm, which is the same order of magnitude as the Na 11 Sn 2 PS 12 sulfide electrolyte (4 × 10 −3 S/cm) reported in [35] and the vacancy-contained Na 3 SbS 4 (3 × 10 −3 S/cm) reported in [22].Therefore, Na 5.5 PS 4.5 Cl 1.5 (x = 0.5, NPSC-5) shows very high values of ionic conductivity among the typical sodium ion conductors that have been reported.
Materials 2024, 17, x FOR PEER REVIEW 7 of 11 steel/NPSC-5/steel configuration was completed in one shot, which can be observed in Video S1.All the obtained samples exhibited linear relationships with ionic conductivity in the range of 25-85 °C. Figure 5a illustrates the temperature dependence of ionic conductivity.All obtained samples exhibited linear relationships between ionic conductivity and reciprocal temperature within the studied temperature range, indicating adherence to the Arrhenius law.By applying the Arrhenius equation (σT = σ0exp(−Ea/kBT), where σ0 references a pre-exponential factor) [49], the activation energy (Ea) of Na6−xPS5−xCl1+x (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) for total conduction was determined from the slope of the plots.In Figure 5b, the composition dependence of the samples' activation energies (Ea) are depicted.AC impedance analysis results revealed that the ionic conductivity of all chlorine-rich Na6−xPS5−xCl1+x samples was higher than that of the primordial Na6PS5Cl (x = 0) sample, with ionic conductivity increasing from x = 0 to 0.5.These findings indicate that a rise in chlorine contents can enhance the ionic conductivity of Na6PS5Cl with a decrease in Ea.Table S1 summarizes the ionic conductivities and Ea values reported in the literature, as well as the values obtained in this study.Compared with the reported Na6PS5Cl (0.01 × 10 −3 S/cm) [40], the modified Na5.5PS4.5Cl1.5 exhibited superior ionic conductivity of 1.2 × 10 −3 S/cm, which is the same order of magnitude as the Na11Sn2PS12 sulfide electrolyte (4 × 10 −3 S/cm) reported in [35] and the vacancy-contained Na3SbS4 (3 × 10 −3 S/cm) reported in [22].Therefore, Na5.5PS4.5Cl1.5 (x = 0.5, NPSC-5) shows very high values of ionic conductivity among the typical sodium ion conductors that have been reported.

Electrochemical Performance of NVP//NPSC-5//Na ASSB.
Na5.5PS4.5Cl1.5 (x = 0.5, NPSC-5) showed the highest ionic conductivity and lowest activation energy in the Na6−xPS5−xCl1+x system, so the electrochemical performance of a laboratory-grade, pressurized NVP//NPSC-5//Na cell with NPSC-5 as a sodium solid electrolyte and commercial Na3V2(PO4)3 (NVP, 99.9%, MTI Corporation, Shenzhen, China) and Na metal foil as cathode and anode, respectively, was determined, as depicted in the schematic diagram in Figure 5a.Since NVP can offer a 1.6 V voltage plateau through the V 3+ /V 2+ redox reaction, which lies within the voltage stability region of the NPSC-5 electrolyte, the galvanostatic charge-discharge of the NVP//NPSC-5//Na ASSB was tested in the potential range of 1.4-1.8V at a current density of 0.05 C. All charge-discharge profiles exhibited a 1.6 V plateau, corresponding to the phase transition of Na3V2(PO4)3/Na4V2(PO4)3 with one Na + intercalation/deintercalation from the host lattices [50].The right figure in Figure 6a shows a physical electronic photograph of an assembled all-solid-state sodium-ion battery that can normally light up an LED light bulb at room 3.3.Electrochemical Performance of NVP//NPSC-5//Na ASSB Na 5.5 PS 4.5 Cl 1.5 (x = 0.5, NPSC-5) showed the highest ionic conductivity and lowest activation energy in the Na 6−x PS 5−x Cl 1+x system, so the electrochemical performance of a laboratory-grade, pressurized NVP//NPSC-5//Na cell with NPSC-5 as a sodium solid electrolyte and commercial Na 3 V 2 (PO 4 ) 3 (NVP, 99.9%, MTI Corporation, Shenzhen, China) and Na metal foil as cathode and anode, respectively, was determined, as depicted in the schematic diagram in Figure 5a.Since NVP can offer a 1.6 V voltage plateau through the V 3+ /V 2+ redox reaction, which lies within the voltage stability region of the NPSC-5 electrolyte, the galvanostatic charge-discharge of the NVP//NPSC-5//Na ASSB was tested in the potential range of 1.4-1.8V at a current density of 0.05 C. All charge-discharge profiles exhibited a 1.6 V plateau, corresponding to the phase transition of Na 3 V 2 (PO 4 ) 3 /Na 4 V 2 (PO 4 ) 3 with one Na + intercalation/deintercalation from the host lattices [50].The right figure in Figure 6a shows a physical electronic photograph of an assembled all-solid-state sodium-ion battery that can normally light up an LED light bulb at room temperature, indicating that NPSC-5 is a pure sodium-ionic conductor material with negligible electronic conductivity and functions as a solid electrolyte.The additional plateau at about 1.42 V shown in Figure 6b can be attributed to the formation of a stable NPSC-5/Na interface during the first cycle, which was similar to reports of other electrochemical investigations sodium ASSBs [2,[51][52][53].The NPSC-5/Na interface resulted in a low coulombic efficiency (58%) during the first cycle (The initial discharge capacity was about 81.9 mAh/g, whereas the charge capacity was only 47.3 mAh/g).The second charge and discharge capacities were 43.1 mAh/g and 42.2 mAh/g, respectively, indicating the rebound of coulombic efficiency (98%).The 10th cycle displayed charge and discharge capacities of 34.3 and 33.7 mAh/g, respectively; nearly 73% of the initial charge capacity was retained, and the coulombic efficiency was maintained at 98%, demonstrating that the synthesized material facilitated the cyclic charging and discharging capabilities of the battery.The energy density of the full cell was about 70 Wh/Kg based on the mass of the active cathode.Further modifications, such as to the tips for battery assembly, compositing the NVP cathode with graphene, or replacing the Na disc with a Na-Sn alloy to improve the performance of the NVP//NPSC-5//Na ASSB, should be analyzed and explored in depth in the future.
temperature, indicating that NPSC-5 is a pure sodium-ionic conductor material with ne ligible electronic conductivity and functions as a solid electrolyte.The additional plate at about 1.42 V shown in Figure 6b can be attributed to the formation of a stable NPS 5/Na interface during the first cycle, which was similar to reports of other electrochemi investigations sodium ASSBs [2,[51][52][53].The NPSC-5/Na interface resulted in a low co lombic efficiency (58%) during the first cycle (The initial discharge capacity was about 8 mAh/g, whereas the charge capacity was only 47.3 mAh/g).The second charge and d charge capacities were 43.1 mAh/g and 42.2 mAh/g, respectively, indicating the rebou of coulombic efficiency (98%).The 10th cycle displayed charge and discharge capacities 34.3 and 33.7 mAh/g, respectively; nearly 73% of the initial charge capacity was retaine and the coulombic efficiency was maintained at 98%, demonstrating that the synthesiz material facilitated the cyclic charging and discharging capabilities of the battery.The e ergy density of the full cell was about 70 Wh/Kg based on the mass of the active cathod Further modifications, such as to the tips for battery assembly, compositing the NVP ca ode with graphene, or replacing the Na disc with a Na-Sn alloy to improve the perfo mance of the NVP//NPSC-5//Na ASSB, should be analyzed and explored in depth in t future.

Conclusions
In summary, halide-rich solid solutions of argyrodite-type Na6−xPS5−xCl1+x were sy thesized and formed solid solutions in the range of 0 ≤ x ≤ 0.5.Increasing Cl -conten effectively reduced the activation barrier and increased the Na-ion mobility, as well electrochemical stability.The optimal solid solution composition of Na5.5PS4.5Cl1.5 exh ited particularly high Na-ion conductivity in the Na6−xPS5−xCl1+x in this work, namely mS/cm at 25 °C, which is more than a hundred times higher than that of Na6PS5Cl with regular composition.Meanwhile, Na5.5PS4.5Cl1.5 is more suitable for high-voltage cathod than Na6PS5Cl due to its wider voltage stability region up to 2 V, and the smooth charg discharge cycling performance of Na3V2(PO4)3//Na5.5PS4.5Cl1.5//Na at room temperatu verifies that the obtained material is a pure sodium-ion conductor and its potential a sodium-ionic solid electrolyte for all-solid-state sodium batteries.To date, the report sulfide electrolyte, Na11Sn2PS12 (4 × 10 −3 S/cm) [35] and vacancy-contained Na3SbS4 (3 × 1 S/cm) [22] have exhibited excellent conductivity.Both references proposed the strategy designing Na vacancy for the improvement of conductivity.Therefore, we suppose th the ionic conductivity of Na5.5PS4.5Cl1.5 could be further promoted by designing Na v cancy in the crystal structure.

Supplementary Materials:
The following supporting information can be downloaded

Conclusions
In summary, halide-rich solid solutions of argyrodite-type Na 6−x PS 5−x Cl 1+x were synthesized and formed solid solutions in the range of 0 ≤ x ≤ 0.5.Increasing Cl − contents effectively reduced the activation barrier and increased the Na-ion mobility, as well as electrochemical stability.The optimal solid solution composition of Na 5.5 PS 4.5 Cl 1.5 exhibited particularly high Na-ion conductivity in the Na 6−x PS 5−x Cl 1+x in this work, namely 1.2 mS/cm at 25 • C, which is more than a hundred times higher than that of Na 6 PS 5 Cl with a regular composition.Meanwhile, Na 5.5 PS 4.5 Cl 1.5 is more suitable for high-voltage cathodes than Na 6 PS 5 Cl due to its wider voltage stability region up to 2 V, and the smooth charge-discharge cycling performance of Na 3 V 2 (PO 4 ) 3 //Na 5.5 PS 4.5 Cl 1.5 //Na at room temperature verifies that the obtained material is a pure sodium-ion conductor and its potential as a sodium-ionic solid electrolyte for all-solid-state sodium batteries.To date, the reported sulfide electrolyte, Na 11 Sn 2 PS 12 (4 × 10 −3 S/cm) [35] and vacancy-contained Na 3 SbS 4 (3 × 10 −3 S/cm) [22] have exhibited excellent conductivity.Both references proposed the strategy of designing Na vacancy for the improvement of conductivity.Therefore, we suppose that the ionic conductivity of Na 5.5 PS 4.5 Cl 1.5 could be further promoted by designing Na vacancy in the crystal structure.
). Comparisons of XRD patterns with those of other related compounds (such as tetragonal Na3PS4, cubic Na3PS4, and NaCl) are displayed in Figure S1, proving that no Na3PS4 or NaCl impurities were included in the NPSC-0 sample.With the increase in x in Na6−xPS5−xCl1+x, the (400) peak gradually shifted to lower 2θ angles, while the (2 22) peak migrated to higher 2θ angles, indicating the formation of solid solutions in Na6−xPS5−xCl1+x.Further introduction of additional chlorine into the Na6−xPS5−xCl1+x structure caused exsolvation of NaCl impurity at x = 0.6 (Figure S2), resulting a decrease in ionic conductivity.Thus, under the present experimental synthesis conditions, it was shown that Na5.5PS4.5Cl1.5 (x = 0.5) is the limit of solid solution formation in the Na6−xPS5−xCl1+x system.In other words, Na6−xPS5−xCl1+x formed solid solutions in the range of 0 ≤ x ≤ 0.5.
. The elemental mapping analyses shown in Figure2b-f indicate the presence and uniform distribution of Na, P, S, and Cl elements within the NPSC-5 sample.
S3.The elemental mapping analyses shown in Figure2b-f indicate the presence and uniform distribution of Na, P, S, and Cl elements within the NPSC-5 sample.
xCl1+x, the voltage stability region widened gradually.When x = 0.1, the stable region was at 1.52 V; when x = 0.2, the stable region shifted to 1.68 V.It kept increasing to 1.79 and 1.91 V in the NPSC-3 and NPSC-4 samples, respectively.The NPSC-5 sample exhibited a maximum stable voltage of 2.02 V, indicating that promoting the Cl content in Na6−xPS5−xCl1+x favors the widening of the operating potential window.

Figure 4
Figure 4 illustrates the complex impedance spectra of the uniaxially cold, isostatic pressed powder samples obtained in the study.The impedance plots exhibit a semicircle
xCl1+x, the voltage stability region widened gradually.When x = 0.1, the stable region was at 1.52 V; when x = 0.2, the stable region shifted to 1.68 V.It kept increasing to 1.79 and 1.91 V in the NPSC-3 and NPSC-4 samples, respectively.The NPSC-5 sample exhibited a maximum stable voltage of 2.02 V, indicating that promoting the Cl content in Na6−xPS5−xCl1+x favors the widening of the operating potential window.

Figure 4
Figure 4 illustrates the complex impedance spectra of the uniaxially cold, isostatic pressed powder samples obtained in the study.The impedance plots exhibit a semicircle